Studies will be made of Mg ions fluxes in Langendorff-perfused rabbit hearts under conditions of hypoxia and during the action of glucagon, epinephrine and cardiac glycosides, using Mg28 as a tracer, and/or employing atomic absorption methods for AV difference measurements of net flux. Studies will also be made of compartmental fluxes of Ca ions using an ion-immobilization technique and non-polar liquid density gradient ultra-centrifugation for fractionation. Short time loading with Ca45 will permit identification of rapidly exchanging fractions of total Ca.